The objectives of the current study are: 1) to analyze critically at both the LM and EM levels the nature and organization of sensory receptors within the dental pulp organ and periodontal ligament of the rat and monkey and, 2) todetermine the central neural projections and synaptic connections of pulpal and periodontal afferent neurons. The first series of experiments will involve the examination of normal tissue and of tissue in which the neural elements have been labeled experimentally by the anterograde transport of HRP. Using the procedures reported by Marfurt and Turner (1), HRP will be injected into either the trigeminal ganglion, mesencephalic nucleus of V, or superior cervical ganglion. The enzyme will be taken up by the ceuronal cell bodies and transported anterogradely to their peripheral endings in the dental tissues. Each population of labeled neural elements will then be analyzed critically to determine its origin, distribution, ultrastructure, and mode of termination within the dental tissues. In the second series of experiments, the central sites of termination of tooth pulp and periodontal afferents will be determined experimentally by the methods of transganglionic transport of HRP. HRP will be placed in either the pulp chamber of vital teeth or in the bases of alveolar sockets following removal of teeth that had been previously devitalized to produce degeneration of the pulpal afferents. The brainstem terminations and synaptic connections of tooth pulp afferents (a presumed source of "pure" nociceptive fibres) will be identified and then compared and contrasted with the central termination sites and synaptic connections of the periodontal ligament afferents. Retrograde transport of HRP to either the superior cervical ganglion or the mesencephalic nucleus of V will also be investigated. The results of the current study should significantly increase our understanding of the peripheral and central mechanisms of dental pain. Knowledge of the neural circuitry subserving tooth pulp pain will contribute to our overall understanding of pain and pain pathways and represents a significant step towards achieving the long term goal of pain control.